A group of diseases including psoriasis, arthritis and Reiters syndrome, and designated for convenience PARS, has been found to occur in association with each other and at elevated frequency in HIV infected individuals. This is a proposal to gather information relevant to their occurrence and natural history, but most importantly to initiate a deeper inquiry into the immunologic mechanisms of disease and the genetic factors encoded by the MHC that may influence their susceptibility. An underlying hypothesis has been proposed, occasioned by the occurrence of the PARS diseases in setting of helper T cell deficiency, namely that they are diseases of immune recognition in which T cells of the CD8 cytotoxic suppressor sublineage recognize determinants on or presented by MHC class I molecule alleles involved in determining disease susceptibility. Much of the proposed is research directed to gaining insight into mechanisms and phenomena relevant to this hypothesis. The association of PARS diseases with HIV infection also offers the opportunity to examine aspects of disease classification. The occurrence of Reiter's syndrome and psoriatic arthritis and the absence of ankylosing spondylitis in AIDS suggests different mechanisms and susceptibility factors in these diseases. It is anticipated that insights into psoriasis and the spondyloarthropathies acquired in this proposal will be applicable to these diseases as they occur in the general population.